In some areas of China, fishery production has been going downhill every year resulting in a depletion of the fish population. The effect of changing natural environmental conditions, and artificial factors are two principal reasons that may account for this downtrend. A fluctuation in natural conditions can cause the destruction of the spawning environment, an increase in predators, and/or a decrease in fish food. Artificial factors include: the use of water birds, mammals, and small meshed nets (methods of fishing that take a considerable quantity of juvenile fish); the damming of rivers which inhibits spawning; and pollution from industries which can be toxic to fish. Chinese freshwater fish culture has made progress in increasing fishery production; however, it has created a problem of an insufficient supply of fish fry. It is important, therefore, to thoroughly understand the breeding habit and fecudity of the fish, in addition to the physical and chemical conditions of the spawning grounds in order to devise conservation measures. For instance, when it is known that grass carp, roach, white bighead and stripped bighead spawn in certain localities in the fast water of a river, then laws can be enacted to prohibit fishing in these areas during the spawning period of each species.